1. Field of the Invention
The present invention relates to the recovery of tungsten from tungsten-bearing ore, and relates in particular to processes in which tungsten-bearing ore is leached to form an aqueous sodium tungstate leach liquor which is treated for recovery of a tungsten-bearing final product such as ammonium paratungstate (APT). More particularly, the present invention relates to reducing the concentration of silica, phosphorus, and fluorine impurities dissolved in sodium tungstate liquor.
A sodium tungstate leach liquor is typically produced by the reaction of ore concentrates in water with, for instance, soda ash to dissolve the tungstate values. The concentrates usually contain impurities which are also dissolved in the leaching reaction. These impurities should be removed from the liquor to as high a degree as possible because they can interfere with subsequent processing steps. Silica interferes with solvent extraction of the leach liquor, and it can contaminate the final tungsten product. Phosphorus and fluorine can interfere with the removal of molybdenum impurities, and can also contaminate the final product. In addition, in some liquor treatment stages reagents are added which contribute minor amounts of impurities that can be tolerated if the concentrations of those impurities in the leach liquor have previously been reduced.
In particular, the leach liquor can contain in solution excessive amounts of silica, phosphorus, and fluorine impurities which should be rejected from solution. By "silica" is meant SiO.sub.2 as well as other silicon-based dissolved impurities including Si(OH).sub.n and SiO.sub.3.sup.-2. By "phosphorus" is meant dissolved phosphorus-based compounds including orthophophates and metaphosphates. By "fluorine" is meant F.sup.- ion as well as other dissolved fluorine-based impurities including SiF.sub.6.sup.-2.
There is a need for a process for rejecting these dissolved impurities from sodium tungstate leach liquor which is effective in a short period of time, and which makes efficient use of reagents and equipment.
This need is particularly acute in the treatment of liquors obtained by high-temperature autoclave leaching of tungsten ores, such as leaching carried out at 200.degree. C. to 240.degree. C. In addition, in some instances the sodium tungstate leach liquor will contain acceptably small or minimal amounts of dissolved silica along with unacceptably high amounts of dissolved phosphorus and fluorine; this condition could arise through leaching the tungsten ore under conditions which minimize the solution of silica, or because of the characteristics of the ore. Thus, a process for rejecting dissolved impurities from sodium tungstate leach liquor is needed which is flexible enough to be effective and economical in treating such leach liquors low in dissolved silica as well as sodium tungstate leach liquors that contain excessive amounts of dissolved silica.
2. Description of the Prior Art
While there have been efforts in the past to reduce the concentration of dissolved substances from aqueous systems, these efforts do not suggest the particular conditions of the present invention for rejecting silica, and the other impurities described above, from basic sodium tungstate leach liquors.
The prior art has recognized the usefulness of magnesium and aluminum compounds in precipitating silica from water (generally from relatively solute-free water), but the art does not suggest the particular process of the present invention. Specifically, Betz, Noll, and Maguire in Ind. and Eng. Chem., Vol. 32, pp. 1320-23 (October 1940) reported that adding aluminum hydroxide to water containing about 20 ppm of dissolved silica rejected silica from solution within a one-hour period, and that rejection is generally favored at lower temperatures within the range 20.degree. C. to 95.degree. C. and specifically at 23.degree. C. Most favorable rejection of silica occurred at a pH of 8.3 to 9.1, but operation at a pH of 7.6 to 8.0 was recommended to avoid dissolving excessive amounts of aluminum. The same authors in Ind. and Eng. Chem., Vol. 32, pp. 1323-29 (October 1940) reported that adding MgO to solutions containing about 22 ppm of dissolved silica rejected silica from solution within about 15 minutes at a pH of 9.7 to 10.6, and that rejection is favored at higher temperatures in the range 30.degree. C. to 95.degree. C. and specifically at 95.degree. C.
U.S. Pat. No. 4,092,400 describes a process for rejecting silica from a sodium tungstate leach liquor. The pH of the liquor is adjusted to between about 8 to 11, and the temperature of the liquor is maintained at about 20.degree. C. to about 30.degree. C. An aluminum or magnesium salt which will form a silica-containing precipitate is then added to the liquor. Compounds specifically said to be useful are aluminum sulfate and sodium aluminate. The patentees state that the precipitation should take about one hour, but can take up to 4 to 6 hours.
Another process comprises adding both 0.08 kg of aluminum sulfate per kg of WO.sub.3 and 0.03 kg of magnesium sulfate per kg of WO.sub.3 to the slurry that is recovered from the tungstate leach reactor, prior to filtration of unleached solids, at a liquor pH of 9 to 9.5 and a temperature of 70.degree. C. to 80.degree. C. This slurry is stirred for 1 hour, solids are filtered off, and then both aluminum sulfate and magnesium sulfate are again added to the filtered liquor in amounts of 0.08 kg aluminum sulfate per kg of WO.sub.3 and 0.03 kg of magnesium sulfate per kg of WO.sub.3, at a liquor pH of 9 to 9.5 and a temperature of 70.degree. C. to 80.degree. C., to form a precipitate which is filtered off. A drawback of this process is that adding aluminum at a pH such as about 9 to 9.5 to a liquor containing appreciable amounts of fluorine impurities can lead to the formation of an aluminum-fluorine complex which is then removed from solution only with great difficulty.